There is a type of nuclear reactor, commonly called a pebble-bed reactor, wherein the nuclear fuel is in the form of relatively small spheres which are formed into a bed in a container having a bottom with a discharge outlet for the pebbles. Unused pebbles are fed to the top of the bed, and with the bed gradually flowing downwardly used or burned-out pebbles are discharged by way of the run-out. Control rods are inserted vertically into the bed as required to control the activity rate and a gas-coolant is flowed through the bed and passages in the container's core.
A small experimental reactor of this kind, the socalled AVR reactor in Juelich, is described in the journal "Atomwirtschaft," 1966, No. 5, on pages 218 to 271. Another pebble-bed reactor of this kind, the so-called THTR reactor in Uentrop, is described in "Atomwirtschaft" of May 1971, on pages 235 to 246. On page 237, reference is made, as a desirable further development of these two reactors, to the so-called Otto Program (once through, then out), in which the fuel elements travel through the core only once from the top to the bottom. In this program, however, the fuel elements of the first core would have to have differently enriched uranium for immediate operation at full load, according to their position in the core, but the fuel elements which need to be refilled according to the burnup can have the same enrichment. The fuel elements withdrawn from the core would then be burned up and could be removed from circulation. The charging in operation and the withdrawal arrangement could then be kept very simple, as the reshuffling of the core and the measurement of the burnup could be eliminated. Appropriate investigations are being carried out at the Nuclear Research Facility Juelich. The further advantages of this Otto Program are described in detail in the German Offenlegungsschrift 21 23 894.
The AVR as well as the THTR reactor have a container forming a cylindrical core which in its lower part changes into a funnel at whose lower tip a discharge arrangement for the fuel spheres is located. The experiments with such a single, central discharge arrangement, however, have shown so far that the desired Otto Program cannot be realized by this discharge arrangement alone in a manner that makes sense from a core-technology point of view. With continuous withdrawal of the fuel spheres with a mean velocity, relative to the core cross section, of about 0.5 cm/d, the velocity of the spheres is about six times greater in the central region of the core than in the outer zones. One would therefore either have to be satisfied with less burnup for the fuel spheres in the central region of the reactor, or these spheres would have to be fed back into the reactor from the top, abandoning thereby the essential advantages of the so-called Otto Program, i.e., burnup which is uniform over the cross section. In order to improve the uniform withdrawal of the spheres over the core cross section, it has already been attempted to arrange in the lower part of the core three or more withdrawal arrangements, several of which are always arranged on a radius. Apart from the fact that the number of these sphere withdrawal devices cannot be increased arbitrarily, because these withdrawal devices cause a considerable expenditure of means design-wise and therefore, substantial costs, tests in that direction have shown that increasing the number of withdrawal devices still does not ensure the desired uniform flow of the spheres.